I believe it's a compositor issue.
Go to - Settings Manager - Window Manager Tweaks - Compositor - Uncheck 'Show shadows under dock windows' .
See link below which illustrates the solution:-
https://arcolinuxd.com/how-to-remove-the...k-on-xfce/
Hope this helps.
Still running 3.8 and lovin' it (running 5.0 as a triple boot)